Ejectors and methods are known in the art for jetting droplets of viscous medium of fluid, e.g. solder paste or adhesive, onto a substrate, e.g. a printed wiring board (PWB), thus forming deposits on the substrate prior mounting components thereon. Such an ejector generally comprises a nozzle space for containing a volume of the viscous medium prior to the jetting thereof, a jetting nozzle communicating with the nozzle space, an impacting device for impacting and jetting the viscous medium from the nozzle space through the jetting nozzle in the form of droplets, and a feeder for feeding the medium into the nozzle space.
The amount, or volume, of the deposited viscous medium at different locations of the substrate may be varied by applying several drops on top of each other, thus forming a larger deposit, or by varying the volume of the jetted droplet by e.g. feeding a larger or smaller volume of the viscous medium into the nozzle space.
High production speed and reliability are factors of interest for the manufacturing of e.g. printed circuit board (PCB) assemblies. In order to increase the production speed, which e.g. can be measured in terms of throughput time or mounted components per hour (cph), the application of viscous medium can be performed “on the fly”, i.e. without stopping for each location on the substrate where viscous medium is to be deposited. The reliability, such as e.g. the accuracy and repeatability of the jetting process, is of interest due to its effects on the performance and the quality of the final product, such as e.g. the PCB assembly. Too small volumes of deposited medium may e.g. lead to dry joints or loosening components, whereas too large volumes of deposited medium may lead to short-circuiting caused by e.g. solder balls, or defective contacts due to contamination of adhesive.
To increase process reliability, an optical or visual inspection such as e.g. manual inspection or Automatic Optical Inspection (AOI) is performed after the viscous medium has been jetted onto the substrate. The relatively limited ability to detect small variations of volume and the time consumption and complexity of such inspection is however a drawback associated with such techniques.
Although such an inspection may provide increased process reliability, there is still a need for providing an ejector and method that would address at least some of the above mentioned issues.